The invention concerns a spark plug comprising a center electrode and a ground electrode, in which the center electrode is arranged relative to the ground electrode so that the spark gap extends between the ground electrode and the center electrode, and the spark gap is surrounded by a wall. The invention further concerns a method of adjusting the spacing of the center electrode relative to the ground electrode of such a spark plug.
Spark plugs pass ignition current into the combustion chamber of an internal combustion engine and there ignite the fuel-air mixture by an ignition spark which flashes over between the electrodes (in the so-called spark gap). High temperatures occur in gas engines involving high power levels and large combustion chambers. In gas engines, an increasing trend is to provide a pre-chamber. The pre-chamber is markedly smaller in comparison with the actual combustion chamber and is separated from the combustion chamber by a wall, and relatively small transfer openings are provided to permit an exchange of fluid between the combustion chamber and the pre-chamber. Actual ignition with the spark plug takes place in the pre-chamber, in which case a comparatively rich fuel-air mixture is ignited there with the spark plug. The ignition flares which are produced in that case pass by way of the transfer openings into the actual combustion chamber and there ignite a comparatively weak fuel-air mixture.
A wall can protect the ground electrode and the center electrode from the high temperatures, and the wall is a good heat conductor. In addition, the electrodes can be extended closer to the insulating body of the spark plug to ensure a faster flow of heat into the spark plug casing. It will be noted, however, that this means that the electrodes are more difficult to access from the exterior. In the state of the art it was therefore necessary, after a certain operating time, to increase the ignition voltage in order to compensate for the spacing between the center electrode and the ground electrode, which becomes larger due to electroerosion. An alternative variant provides that the spark gap is not arranged parallel to the longitudinal axis of the spark plug but perpendicularly thereto in order to achieve access to the spark gap at least from the end of the spark plug. The disadvantage of that procedure however is that, in the case of such spark plugs, a plurality of ground electrodes are required as otherwise the ignition spark extends asymmetrically and the spark gap which is parallel to the longitudinal axis of the spark plug is undesirable.